1. Field of the Invention
The present invention relates to communications systems, and, in particular, to receivers and transmitters having polyphase structure.
2. Description of the Related Art
In many communications schemes, multiple signal channels are processed within a transmitter for transmission as a single signal stream to a receiver which is then able to process the received signal stream to recover the individual signal channels. Examples of such communications schemes are analog AMPS (American Mobile Phone System) and the time division multiple access (TDMA) industry standard (IS)-54 scheme. In order to reduce overall equipment cost, such channelizing transmitters and receivers are often designed to use as much common equipment as possible for processing all of the signal channels. One advantage of these so-called "block processing" techniques is that there will typically not be significant additional costs incurred as extra signal channels are added to the system.
Referring now to FIG. 1, there is shown a representation of an example of the filter specification requirements for AMPS and TDMA (IS-54). According to those specification, all candidate transfer functions should lie outside regions identified as 102 and 104 in FIG. 1. One aspect of the present invention is to provide channelizing transmitters and receivers that satisfy the requirements for communications schemes such as AMPS and TDMA (IS-54). Structures in accordance with the present invention may be implemented with fewer coefficients--and are therefore less complex and less expensive--than conventional designs.
Further aspects and advantages of this invention will become apparent from the detailed description which follows.